1. Technical Field
Embodiments of the present invention relate generally to semiconductor memory devices.
2. Related Art
A flash memory device is a type of semiconductor memory and is organized by including a plurality of memory cell blocks. Each memory cell block (or memory block) is normally composed of a plurality of cell strings in which a plurality of memory cells are coupled in series, a plurality of bit lines, a plurality of word lines, drain selection transistors coupled between the cell strings and bit lines, and source selection transistors coupled between the cell strings and a common source line. A group of memory cells sharing a word line may constitute a page. Memory cells included in a unit memory block are commonly formed in a P-well of a semiconductor substrate. Further, a pass transistor circuit may be included to provide voltages to the memory block in the flash memory device. The pass transistor circuit may be composed from a drain selection high-voltage transistor, a source selection high-voltage transistor, and a cell selection high-voltage transistor.
In the flash memory device with such a configuration, in order to program data into memory cells, erasing is first carried out on all memory cells and then programming is performed only to selected memory cells. While programming is conducted on a page-by-page basis, erasing is performed on a memory block-by-memory block basis because all of the memory cells are sharing the P-well.
FIG. 1 graphically depicts threshold voltage distributions of memory cells resulting from programming and erasing operations of a general semiconductor memory device.
Referring to FIG. 1, erased memory cells having lower threshold voltages than a read voltage READ become programmed memory cells having higher threshold voltages than the read voltage READ as a result of a programming operation. On the other hand, programmed memory cells having higher threshold voltages than the read voltage READ become erased memory cells having lower threshold voltages than the read voltage READ as a result of an erasing operation.
However, even when only the selected pages should be erased, a flash memory device designed to erase data on a memory block-by-memory block basis may operate to erase the entire memory block, including the selected pages and unselected pages. As a result, the efficiency of data management in the flash memory device is reduced.